1. Field of the Invention
The present invention relates to a polishing composition for polishing alumina disks, polishing substrates having silica surfaces and semiconductor wafers, comprising a stable aqueous silica sol containing moniliform colloidal silica particles having a ratio (D1/D2) of a particle diameter D1 nm (as measured by dynamic light scattering method) to a mean particle diameter D2 (as measured by nitrogen absorption method) of 3 or more, wherein D1 is between 50 to 800 nm and D2 is between 10 to 120 nm, said moniliform colloidal silica particles being composed of spherical colloidal silica particles and a metal oxide-containing silica bond which bonds these spherical colloidal silica particles together, wherein the spherical colloidal silica particles are linked together in rows in only one plane by observation through an electron microscope, and further wherein said polishing composition contains 0.5 to 50% by weight of said moniliform colloidal silica particles. Hereafter, a stable aqueous silica sol containing moniliform colloidal silica particles is referred to as a moniliform silica sol.
More particularly, the moniliform silica sol is characterized by the shape of its colloidal silica particles and the polishing method of the present invention can provide highly accurate smooth polished surface efficiently and hence it is useful as a finish polishing method. Here, polishing of an aluminum disk means polishing a surface of the substrate itself of a magnetic memory disk composed of aluminum or its alloy, or a surface of a plating layer such as Nixe2x80x94P, Nixe2x80x94B, etc., in particular a hard layer of non-electrolysis nickel-phosphorus (Nixe2x80x94P) plating having a composition of 90 to 92% Ni and 8 to 10% P and an aluminum oxide layer provided on the substrate. Polishing of a substrate having silica on its surface means polishing a surface layer on a substrate containing 50% by weight of silica or more, for example, polishing rock crystal, quartz glass for photomasks, SiO2 oxide layer for semiconductor devices, crystallized glass made hard disks, and aluminosilcate glass or soda lime glass reinforced glass made hard disks. Polishing semiconductor wafers means polishing semiconductor wafers made of elemental silicon, compound semiconductor wafers made of gallium arsenide, gallium phosphide, indium phosphide, etc.
Since the polishing composition of the present invention can efficiently provide highly accurate smooth polished surfaces, it is useful In precision polishing of wiring metals such as copper and aluminum in semiconductor multilayer interconnection board, of nitride layer and of carbide layer, and in finish polishing of monocrystals such as sapphire, lithium tantalate, and lithium niobate, GHR magnetic heads. etc.
2. Description of the Related Art
An aqueous silica sol generally has the property that it is converted from a state where it has a low viscosity to a gelled state through a state where it has a high viscosity. Therefore, if the SiO2 content is the same, the lower viscosity product is evaluated as being more stable than the higher viscosity product. An aqueous silica sol shows a lower viscosity the closer the shape of colloidal silica particle contained therein is to regular sphere. For this reason, aqueous sols composed of highly stable, spherical colloidal silica particles have been conventionally used partly in finish polishing of aluminum disks, glass disks, quartz glass for photomasks, rock crystal, siliceous substrates such as SiO2oxide film of semiconductor devices, monocrystals such as semiconductor wafers, sapphire, lithium tantalate. lithium niobate, etc., and MR magnetic heads. etc. However, although such silica sols give highly flat polished surfaces, it has been pointed out that they have the defect of slow polishing speed. Accordingly, to increase polishing speed, efforts have been made to change the shape of colloidal silica particles. For example, JP-A-7-221059 describes use of colloidal silica particles having a lopsided shape with a longer diameter being 7 to 1,000 nm and a ratio of shorter diameter/longer diameter being 0.3 to 0.8 in polishing semiconductor wafers results in an increased polishing speed. However, no proposal has been made for improving polishing properties such as polishing speed by using of the moniliform colloidal silica sol.
As a polishing composition for aluminum disks composed of silica (silicon dioxide), water, and a polishing accelerator, U.S. Pat. No. 4,959,113 discloses a polishing method for nickel plated aluminum disks with a polishing composition comprising water, an abrasive (silicon dioxide, aluminum oxide, cerium oxide, etc.) and a salt composed of iron ion as a cation and nitrate ion or sulfate ion as an anion. U.S. Pat. No. 5,997,620 (related patent: JP-A-10-204416) discloses a polishing composition for memory hard disks, containing water and an abrasive (silicon dioxide, aluminum oxide, cerium oxide, etc.) and soluble iron compound.
The present invention is to provide a polishing composition having excellent polishing properties for aluminum disks, glass hard disks, quartz glass, rock crystal, SiO2 oxide film of semiconductor devices, elemental silicon semiconductor wafers, and compound semiconductor wafers. Further, the present invention is to provide a polishing composition for aluminum disks having an increased polishing speed by adding to the above polishing composition of the invention one or more of iron compounds selected from the group consisting of iron (III) nitrate, iron (III) chloride, iron (III) sulfate, and potassium iron (III) sulfate [KFe(SO4)2].
The moniliform silica sol as a constituent substance of the polishing composition of the present invention has a SiO2, concentration of 50% by weight or less and is stable. The shape of the colloidal silica particle dispersed in a liquid medium of the silica sol of which a particle diameter (D1 nm) measured by dynamic light scattering method is 50 to 800 nm is featured as follows. When observed on electron microscope, the particles are composed of spherical colloidal particles and silica bounding these spherical colloidal particles, and the colloidal silica is moniliform in shape, which are linked in rows in only one plane and the degree of linking, i.e., a D1/D2value, the ratio of the above-described D1 to a mean particle diameter D2 (particle diameter measured by a nitrogen absorption method) is 3 or more.
The moniliform silica sol is a stable aqueous silica sol containing moniliform colloidal silica particles having a ratio (D1/D2) of a particle diameter D1 nm (as measured by dynamic light scattering method) to a mean particle diameter D2 (as measured by nitrogen absorption method) of 3 or more, wherein D1 is between 50 to 800 nm and D2 is between 10 to 120 nm, said moniliform colloidal silica particles being composed of spherical colloidal silica particles and a metal oxide-containing silica bond which bond which bonds these spherical colloidal silica particles together, wherein the spherical colloidal silica particles are linked together in rows in only one plane by observation through an electron microscope. The silica sol has 5 to 50% by weight of moniliform colloidal silica particles.
The moniliform silica sol can be efficiently obtained by a production method comprising the steps of (a), (b), (c), and (d) below:
(a) the step of adding an aqueous solution containing a water-soluble divalent (II) or trivalent (III) metal salt singly or in admixture to an aqueous colloid solution of activated silicic acid or acidic silica sol having a mean particle diameter of 3 to 8 nm, having an SiO2 concentration of 0.5 to 10% by weight and a pH of 2 to 6, in an amount of 1 to 10% by weight of the metal oxide (MO in the case of the divalent metal (II) or M2O3 in the case of the trivalent metal (III), where M represents a divalent or trivalent metal atom, and O represents an oxygen atom) based on the SiO2 in the aqueous solution of the activated silicic acid or the acidic silica sol, and mixing,
(b) the step of mixing the mixture (a) obtained in the step (a) with acidic spherical silica sol having a mean particle diameter of 10 to 120 nm and a pH of 2 to 6, in an amount such that a ratio A/B (weight ratio) of the content (A) of silica derived from the acidic spherical silica sol to the content (B) of silica derived from the mixture (a) is 5 to 100 and that total silica content (A+B) of a mixture (b) obtained by mixing the acidic spherical silica sol and the mixture (a) is an SiO2, concentration of 5 to 40% by weight in the mixture (b),
(c) the step of mixing the mixture (b) obtained in the step (b) with an alkali metal hydroxide, a water-soluble organic base or water-soluble silicic acid salt thereof such that pH is 7 to 11 and mixing, and
(d) the step of heating the mixture (c) obtained in the step (c) at 100 to 200xc2x0 C. for 0.5 to 50 hours.
The shape of the colloidal silica particle that constitutes the moniliform silica sol can be seen by photograph using an electron microscope. Many colloidal silica particles present in the silica sol are not limited to the same shape but present themselves commonly in moniliform. The many colloidal silica particles are roughly grouped into four groups, i.e., those linked substantially in a straight line, those linked in a curved state, those linked in a branched state, and those linked in a cyclic state. It is difficult to express their fractions in accurate figures. However, photographic observations indicates that the fractions of those linked in a curved state and those linked in a branched state are highest and these types are dominant.
Observing a single particle, it consists of spherical colloidal silica particles corresponding to xe2x80x9cbeadsxe2x80x9d of xe2x80x9ca necklacexe2x80x9d and silica as a linking part corresponding to the xe2x80x9cthreadxe2x80x9d of xe2x80x9ca necklacexe2x80x9d. That the colloidal silica particle is not in an elongated form but in a moniliform is due to a difference in the method of producing silica sol and the degree of moniliform (degree of linking of spherical colloidal silica particles) varies depending on the production conditions and the degree may be determined depending on the empirical rule of production.
Most of the colloidal silica particles in the silica sol prepared by a predetermined method under predetermined conditions have a degree of linking controlled within a certain range. The colloidal silica particles of the silica sol obtained by this method are composed of spherical colloidal silica particles of a mean particle diameter of 10 to 120 nm. Most of the colloidal silica particles in the silica sol do not have a fixed length. However, according to photographic observation, the length is 5 times or more the diameter of the sphere and usually those particles having lengths 5 to 10 times the diameter are dominant.
The colloidal silica particle that constitutes the moniliform silica sol has another feature that the moniliform linking occurs in the same plane. Since they have linking in the same plane even if they are curved or branched, all the particles are laid in the same plane at a height corresponding to the height of the spherical colloidal silica particle even if they are different in shape unless they are superimposed. In electron micrograph, the colloidal silica particles in the moniliform silica sol tend to be superimposed one on another and It is difficult to discern one end from the opposite end of one particle so that it is difficult to measure the length of that particle. However, it can be judged by photography that existence of linking of colloidal silica particles in three-dimensional directions makes a ball-like agglomeration. It can be said that basically there is no long linking in three-dimensional direction although one or two linkings occur in three-dimensional direction.
It is not appropriate to express the size of the colloidal silica particle that constitutes such a moniliform silica sol as a length presumed from an electron micrograph but it is appropriate to express it as a value measured by a dynamic light scattering method that allows measurement of the size of a particle in terms of a dimension that corresponds to length. The method for measuring particle diameter by a dynamic light scattering method is explained in Journal of Chemical Physics, vol. 57, No. 11 (December, 1972) p.4814 and can be performed using a commercially available apparatus called model N4 manufactured by Coulter in U.S.A. The particle diameter (D1) as the size of colloidal silica particle that constitutes the moniliform silica sol is 50 to 800 nm as measured by the dynamic light scattering method.
The mean particle diameter (D2) of the spherical colloidal silica particle that constitutes the moniliform colloidal silica particle is given from a specific surface area S m2/g measured usually by a nitrogen absorption method by the equation D2=2720/S (nm).
Therefore, the ratio D1/D2 of the particle diameter D1 nm measured by the above dynamic light scattering method to the above D2 nm means the degree of linking (degree of length) of the moniliform colloidal silica particles. The D1/D2 value of the moniliform silica sol is 3 or more, usually 4 to 20.
The silica that bonds the spherical colloidal silica particles that constitutes the moniliform silica sol is substantially amorphous silica although it contains 0.5 to 10% by weight of divalent (II) or trivalent (III) metal oxide based on SiO2 in the silica sol used in bonding and deriving from the silica sol manufacturing method.
The moniliform silica sol has an SiO2 concentration of 50% by weight or less, preferably 5 to 40% by weight, and has the spherical colloidal silica particles linked in a moniliform in the same plane so that the viscosity of the moniliform silica sol is higher than that of the spherical silica sol. The viscosity of the silica sol is higher, the higher the degree of linking of the spherical colloidal particles and the content of SiO2 in the silica sol are. With the above SiO2, concentration of 50% by weight or less, the silica sol has a viscosity of about several mPa.s to about 1, 000 mPa.s at room temperature. The silica sol is highly stable at such a high viscosity and causes no precipitation or gelation during its storage.
The content of the moniliform colloidal silica particles in the polishing composition of the present invention is 0.2 to 50% by weight, preferably 1 to 30% by weight, as SiO2 concentration. If the SiO2, concentration is less than 0.2% by weight, the effect of polishing is small, and if the SiO2 concentration is higher than 50% by weight, the sol is unstable.
In polishing aluminum disks, the moniliform silica sol can be used as it is as an alkaline sol. Alternatively, it can also be used as a sol obtained by treating the alkaline silica sol by cation exchange resin or as an acidic sol obtained by addition of a water-soluble acidic substance such as hydrochloric acid, sulfuric acid, nitric acid, acetic acid, phosphoric acid or the like.
As the polishing accelerator, the content of iron compound such as iron (III) nitrate, iron (III) chloride, iron (III) sulfate, or potassium iron (III) sulfate is preferably 0.01 to 5.0% by weight as Fe2O3 concentration. If the Fe2O3 concentration is less than 0.01% by weight, the effect of polishing acceleration is low, and if more than 5.0% by weight, the silica sol is unstable.
In polishing glass hard disks, the moniliform silica sol can be used as it is as an alkaline aqueous silica sol. Alternatively, it can also be used as a sol obtained by treating the alkaline silica sol after cation exchange treatment or as an acidic sol obtained by addition of a water-soluble acidic substance such as hydrochloric acid, sulfuric acid, nitric acid, acetic acid, phosphoric acid or the like.
In polishing silicon semiconductor wafers, the moniliform silica sol can be used as an alkaline aqueous silica sol by addition of an alkaline substance such as sodium hydroxide, a water-soluble amine such as monoethanolamine, a quaternary ammonium base and its salt, alkali metal salt or the like, which is a known polishing accelerator. Further, in polishing compound semiconductor wafers, the alkaline aqueous silica sol may be used as a sol obtained by treating the alkaline aqueous silica sol by cation exchange resin or as an acidic sol obtained by addition of a water-soluble acidic substance such as hydrochloric acid, sulfuric acid, nitric acid, acetic acid, phosphoric acid or the like.
Further, the polishing composition of the present invention may contain alumina, zirconia, zirconium silicate, mullite, cerium oxide, iron oxide, chromium oxide, titanium oxide, tin oxide, etc. It may also contain hydrated oxides such as aluminum hydroxide, boehmite, goethite, etc. and non-oxides such as diamond, boron nitride, silicon nitride, silicon carbide, etc.
Also, water-soluble alcohols generally added to polishing compositions, such as ethanol, propanol, ethylene glycol, and propylene glycol, sodium alkylbenzenesulfonate, celluloses such as hydroxyethyl cellulose may also be added to the polishing composition of the present invention.